title: Oligomeric structure of type I and type II transforming growth factor beta receptors: homodimers form in the ER and persist at the plasma membrane.
abstract: Transforming growth factor beta (TGF-beta) signaling involves interactions of at least two different receptors, types I (TbetaRI) and II (TbetaRII), which form ligand-mediated heteromeric complexes. Although we have shown in the past that TbetaRII in the absence of ligand is a homodimer on the cell surface, TbetaRI has not been similarly investigated, and the site of complex formation is not known for either receptor. Several studies have indicated that homomeric interactions are involved in TGF-beta signaling and regulation, emphasizing the importance of a detailed understanding of the homooligomerization of TbetaRI or TbetaRII. Here we have combined complementary approaches to study these homomeric interactions in both naturally expressing cell lines and cells cotransfected with various combinations of epitope-tagged type I or type II receptors. We used sedimentation velocity of metabolically labeled receptors on sucrose gradients to show that both TbetaRI and TbetaRII form homodimer-sized complexes in the endoplasmic reticulum, and we used coimmunoprecipitation studies to demonstrate the existence of type I homooligomers. Using a technique based on antibody-mediated immunofluorescence copatching of receptors carrying different epitope tags, we have demonstrated ligand-independent homodimers of TbetaRI on the surface of live cells. Soluble forms of both receptors are secreted as monomers, indicating that the ectodomains are not sufficient to mediate homodimerization, although TGF-beta1 is able to promote dimerization of the type II receptor ectodomain. These findings may have important implications for the regulation of TGF-beta signaling.
authors.affiliation: Department of Neurobiochemistry, The George S. Wise Faculty of Life Sciences, Tel Aviv University, Tel Aviv 69978, Israel.
authors.forename: L
authors.initials: L
authors.lastname: Gilboa
authors.forename: R G
authors.initials: RG
authors.lastname: Wells
authors.forename: H F
authors.initials: HF
authors.lastname: Lodish
authors.forename: Y I
authors.initials: YI
authors.lastname: Henis
mesh_terms: D030201:Activin Receptors, Type I; D000818:Animals; D001665:Binding Sites / Q000235:genetics; D019556:COS Cells; D002460:Cell Line; D002462:Cell Membrane / Q000737:chemistry / Q000378:metabolism; D004274:DNA, Recombinant / Q000235:genetics; D019281:Dimerization; D004229:Dithiothreitol / Q000494:pharmacology; D004721:Endoplasmic Reticulum / Q000737:chemistry / Q000378:metabolism; D005455:Fluorescent Antibody Technique; D007150:Immunohistochemistry; D008024:Ligands; D011233:Precipitin Tests; D017346:Protein Serine-Threonine Kinases / Q000032:analysis / Q000737:chemistry*; D000077293:Receptor, Transforming Growth Factor-beta Type I; D000077294:Receptor, Transforming Growth Factor-beta Type II; D011956:Receptors, Cell Surface / Q000737:chemistry / Q000378:metabolism; D018125:Receptors, Transforming Growth Factor beta / Q000032:analysis / Q000737:chemistry* / Q000235:genetics
chemical_list: D004274:DNA, Recombinant; D008024:Ligands; D011956:Receptors, Cell Surface; D018125:Receptors, Transforming Growth Factor beta; D017346:Protein Serine-Threonine Kinases; D030201:Activin Receptors, Type I; D000077293:Receptor, Transforming Growth Factor-beta Type I; D000077294:Receptor, Transforming Growth Factor-beta Type II; D004229:Dithiothreitol